Filter fabrics used for cleaning air include filter fabrics for depth filtration and filter fabrics for surface filtration, and dust collectors use the filter fabrics for surface filtration. In the case of surface filtration, dust is collected on the surface of a filter fabric, to form a dust layer on the surface of the filter fabric, and dust is successively collected by the dust layer. When the dust layer grows to have a certain thickness, it is removed from the surface of the filter fabric by means of air pressure, and the operation to form a dust layer on the surface of the filter fabric is repeated again.
The fibers constituting the filter fabrics used for filtering the high temperature exhaust gases emitted from refuse incinerators, coal boilers, metal blast furnaces and the like include polyphenylene sulfide (hereinafter abbreviated as PPS) fibers, meta-aramid fibers, fluorine-based fibers, polyimide fibers, etc. respectively excellent in heat resistance and chemical resistance, and they are formed into nonwoven fabrics to be used as filter fabrics. Among them, PPS fibers are excellent in hydrolytic resistance, acid resistance and alkali resistance, and widely used as bag filters for dust collection of coal boilers.
Meanwhile, environmental regulations tend to be more severe in the world, and especially the PM2.5 regulations that are going to be enacted in USA may be applied also in Japan. To meet this situation, it is demanded that a filter with a higher dust-collecting efficiency and excellent dimensional stability at higher temperatures is available. The conventional electrostatic precipitators and cyclone separators cannot catch up with the demand, and it is desired that a nonwoven fabric filter with higher functions is available.
Moreover, when such filter fabrics are used, for example, in refuse incinerators, the chemical deterioration caused by high temperature exhaust gases and the chemicals, etc. contained in the exhaust gases takes place, and at the same time, in addition, the physical deterioration such as the wearing due to the contact with the cage and the flexing fatigue respectively caused by the pressure loss during exhaust gas filtration and by the pulse jetting during back-pulse cleaning also takes place. Therefore, the filter fabrics used in bag filters are required to have mechanical strength such as abrasion resistance, in addition to the above-mentioned heat resistance, chemicals resistance and hydrolytic resistance.
JP10-165729A proposes a filter cloth in which PPS fibers with a single fiber fineness of 1.8 d (2.0 dtex) or less are disposed as the surface layer. This method is certainly good in dust release characteristics and dust collection performance, but since it is insufficient in the stiffness at high temperature and abrasion resistance, it has such a problem that the filter cloth is physically progressively deteriorated and broken while it is used.
Furthermore, JP9-075637A proposes a filter cloth for bag filters, comprising a felt composed of fluorine staple fibers, in which the staple fibers in an upper layer portion are different in diameter from those in a lower layer portion. In this invention, a layer composed of fine fibers is disposed on the upstream side while thick fibers are disposed on the downstream side, to provide a filter cloth gently increasing in pressure loss, hence having a longer life. However, since fluorine fibers are relatively low in stiffness, there is a fear that the stiffness especially at high temperature declines. Furthermore, since fluorine fibers are insufficient in abrasion resistance, there is such a problem that the filter cloth is broken due to physical deterioration during use.
Moreover, in order to enhance the mechanical strength of a filter, JP7-16570B proposes a filter, in which a very fine fiber layer and a felt substrate layer are integrated by means of needle punching treatment to gradually decrease the distribution of the fibers capable of being made very fine, from the front surface to the back surface, and subsequently high pressure water jet punching is used to divide the fibers capable of being made very fine, to make the fibers very fine. This method can certainly enhance the dust-collecting efficiency owing to the very fine fibers of the felt surface layer, but has a problem that increased processing steps raise the processing cost.
Moreover, JP2000-334228A proposes a heat resistant filter cloth formed by laminating and integrating a lap consisting of, for example, polytetrafluoroethylene fibers and PPS fibers, a woven fabric composed of PPS fibers and a woven fabric composed of glass fibers in this order. This method is intended to enhance the mechanical strength by laminating a woven fabric composed of glass fibers, but has such problems that since the glass fibers are low in the resistance against alkaline chemicals, a heat resistant filter cloth using the chemical resistance of PPS fibers cannot be provided and that the strength decline during wet heat treatment (autoclave treatment) is very large.
JP2000-140530A proposes a filter cloth for high performance bag filters, obtained by blending, for example, PPS fibers and at least one or more kinds of fibers selected from polyimide fibers, polyamideimide fibers, polytetrafluoroethylene fibers and glass fibers. However, this invention uses PPS fibers with a 180° C. dry heat shrinkage rate of 3% or more, and does not improve the dimensional stability of a nonwoven fabric filter using PPS fibers. Furthermore, the filter cloth has a problem that when PPS fibers and other fibers are blended, blending irregularity is likely to occur.
JP2002-204909A proposes a filter cloth in which fluorine fibers are entangled with the surface of a heat resistant substrate. This method is certainly good in dust release characteristics, in preventing the penetration of particles into the filter cloth and in reducing the pressure loss during the operation of the dust collector. However, the filter cloth has such problems that since the air permeability in the initial state is so low as to increase the initial pressure loss, the life of the filter cloth is shortened, and that the capability of treating the exhaust gas greatly declines. Moreover, it has a problem that plural processing steps of laminating a web composed of staple fibers of polytetrafluoroethylene and entanglement treatment are necessary after preparation of a heat resistant felt substrate. Furthermore, it has a problem that the laminated web layer composed of staple fibers of polytetrafluoroethylene is separated by impact during use as a bag filter.